tools/kvm_stat: simplify initializers

Simplify a couple of initialization routines:
* TracepointProvider and DebugfsProvider: Pass pid into __init__() instead
  of switching to the requested value in an extra call after initializing
  to the default first.
* Pass a single options object into Stats.__init__(), delaying options
  evaluation accordingly, instead of evaluating options first and passing
  several parts of the options object to Stats.__init__() individually.
* Eliminate Stats.update_provider_pid(), since this 2-line function is now
  used in a single place only.
* Remove extra call to update_drilldown() in Tui.__init__() by getting the
  value of options.fields right initially when parsing options.
* Simplify get_providers() logic.
* Avoid duplicate fields initialization by handling it once in the
  providers' __init__() methods.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>